Disc storage systems are known in the art and are used to store information for later retrieval. Such disc storage systems include a rotating disc which carries information thereon. A transducing head is positioned over a surface of the disc while the disc rotates at high speed. The head is carried on a slider which is designed to “fly” just over the surface of the rotating disc, the separation between the slider and the disc being maintained by a film of air (air bearing). The head is used to write information onto the disc or read information from the disc when the slider flies over the disc surface. Such information may be, for example, magnetically or optically encoded on the disc surface.
Increased storage density is becoming increasingly important. One technique known to increase storage density is to decrease the “fly height” of the head. Fly height is defined as the distance between the disc surface and the head or slider during operation of the storage system. A reduced fly height allows information to be written or read back more precisely and such information can be stored in a smaller area (i.e., at a higher density).
During its flight, the slider undergoes continuous vibration, pitch and roll as the topography of the disc changes beneath the slider. If the quality of the disc or the slider is poor, occasional rubbing or sharp contact may occur between the disc and the slider, leading to damage to the head or to the disc, and possibly the loss of valuable data. Various attempts have been made to provide increased assurance that such undesirable contact between the slider and the disc does not occur. These techniques include rigid manufacturing and quality assurance specifications for both the disc and the air bearing surface of the slider. However, in disc drives employing sliders operating at extremely low fly heights, the air bearing itself cannot maintain proper clearance with the disc. This is because of several factors including mean losses from disc glide avalanche, carbon overcoats, pole tip recession and lubricant coating on the disc surface that leave insufficient space for the air-bearing surface to provide the necessary fly height variation. U.S. Pat. No. 5,377,058 entitled “FLY HEIGHT SERVO CONTROL OF READ/WRITE HEAD SUSPENSION” describes a technique for fly height control of a slider flying at a very low fly height. The technique described in U.S. Pat. No. 5,377,058 relates to correcting the fly height for both radial and circumferential error as the head changes radial and circumferential position relative to the disc surface. In general, this technique includes sensing the fly height in real time, comparing the sensed fly height against a reference fly height, and adjusting the suspension system of the slider in accordance with the departure of the sensed fly height of the slider from the reference fly height. However, sensing the fly height in real time during disc read/write operations may interfere with the read/write process because the fly height sensor and the read/write head are usually in close proximity with each other. In addition, dynamically sensing fly height during the reading and writing of data adds complexity to the read/write process.
The present invention addresses these problems, and offers other advantages over the prior art.